If you need to customize the standard way of widget creation you can create your own implementation of the FormToolkit.
To do so you have either to subclass the RCPFormToolkit (or subclasses) or to implement IFormToolkitEx.
RCPForms provides built in support for eclipse themes. E.g. you can change default dialog font used in forms.
If you don't want to enhance the FormToolkit you can also use the FormToolkit.adapt()-Method to get the right fonts, backgroundcolors...